The present invention relates to an improvement in copying apparatus for the removal of the final image support surface from the initial image support surface after the transfer of the image.
In a transfer electrostatographic process such as conventional transfer xerography, in which an image pattern of dry particulate unfused toner material is electrostatically transferred to a final image support surface (the copy sheet) from an initial image support surface (the charged photoreceptor surface developed with toner), the transferred toner is typically only loosely adhered to the final support surface and is easily disturbed by the subsequent necessary process of stripping the final support surface away from the initial support surface unless such stripping is carefully controlled. In the transfer station the copy sheet must be maintained at all times in accurate registration with the toner image to be transferred, and not shifted relative to the imaging surface as it is stripped, or the unfused toner image may be smeared. The photosensitive imaging surface is typically easily scratched or abraded sufficiently to produce visible copy defects by direct contact of a mechanical stripping element therewith. The copy sheets themselves are thin, relatively delicate and may be highly variable in condition, humidity, material, weight, etc.. Further, the stripping of the copy sheet from the photosensitive surface is resisted by the electrostatic attraction between any transfer charge remaining on the copy sheet and the photoreceptor. Stripping can also be resisted or made more difficult by pre-existing curls in the lead edge of the copy sheet. Thus, it may be seen that the stripping system of an electrostatographic copying system has difficult challenges to overcome these obstacles of electrical transfer charge tacking of the copy sheet to the photoreceptor, etc., with severe limitations on the type of sheet stripping mechanism which can be utilized without damaging the initial imaging surface or disturbing the toner image before or after transfer.
The drum or belt on which the initial imaging surface moves inherently fluctuates or shifts in position at the stripping area during operation, perpendicularly to its desired movement direction, due to mechanical tolerances in the imaging surface and its supports, and bearings. This eccentricity in the rotation of a cylindrical drum photoreceptor is known as run-out. These imaging surface position fluctuations typically exceed the thickness of the copy sheet thereon. Thus, a fixed or rigid stripping system relying on mechanically catching a sheet with a fixed position stripping edge would not be practical in a conventional copier. Thus, an effective mechanical stripping system requires some means for sensing and following such imaging surface fluctuations.
In xerography, the toner image transfer is most commonly achieved by electrostatic force fields created by D.C. charges applied to or adjacent the back of the copy sheet while the front side of the copy sheet contacts the toner bearing photoreceptor surface. These transfer fields must be sufficient to overcome the forces holding the toner onto the photoreceptor and to attract a substantial portion of the toner onto the copy sheet. The transfer fields are generally provided in one or two well known ways; by ion emission of D.C. charges, from a transfer corona generator, deposited onto the back of the copy paper, or by a D.C. biased transfer roller or belt rolling along the back of the paper, and holding it against the photoreceptor. In either case the copy sheet must be held in registration with, and moved together with, the imaging surface in order to transfer a registered and unsmeared image. Particularly in the conventional transfer accomplished by D.C. corona charges applied to the back of the copy sheet, these transfer charges also provide a substantial "tacking" force which electrostatically holds the copy sheet down against the imaging surface for the movement of the copy sheet therewith.
Thus, a particularly difficult problem in modern xerographic transfer systems is the reliable and consistent stripping of the copy sheet off of the imaging surface after the transfer of the image has been accomplished. Due to practical space and time constraints, this must generally be done as closely as possible after the transfer step, yet without disturbing the transferred toner image on the copy sheet.
Various stripping systems have been utilized in the prior art. One such system is an air puffer applying a jet of air towards the lead edge of the copy sheet to initiate its separation from the imaging surface, as described in U.S. Pat. No. 3,062,536, issued Nov. 6, 1962, to J. Rutkus, Jr., et al.. Another is a vacuum stripping system as shown, for example, in U.S. Pat. No. 3,885,785, issued May 27, 1975, to R. A. Burkett, et al..
Various mechanical stripping systems are known using stripping fingers for catching the lead edge of the copy sheet and stripping the sheet from the photoreceptor. An example of an effective commercial mechanical stripping finger system is disclosed in U.S. Pat. No. 3,578,859, issued May 18, 1971, to W. K. Stillings.
The present invention relates to such mechanical or direct sheet contact sheet stripping, with improved protection for the photoreceptor imaging surface, yet without requiring large, complex, or expensive mechanical, electrical or pneumatic systems.
Another type of stripping system, which desirably may be used in cooperation with the present system, is a self-stripping system utilizing a detacking corotron, taught in U.S. Pat. No. 3,870,515, issued Mar. 11, 1975, to Norbett H. Kaupp. This system utilizes the self-straightening tendency of the copy sheet to continue along a linear path when the imaging surface curves away from this path at a stripping area in combination with a detacking corotron to remove most of the tacking charge. However, it is not necessarily desirable to remove all of the transfer charge on the copy sheet to aid in stripping, since that may also reduce the electrostatic retention of the toner image to the copy sheet.
The Stillings patent cited above also discloses a vacuum manifold sheet guide system closely adjacent the photoreceptor and forming a part of the stripping system after stripping of the lead edge has been initiated. That is, once a lead edge area of the sheet has been initially stripped, it may be captured by a downstream sheet transport and the remainder or body of the sheet can be removed by that transport. Such subsequent sheet transport systems may also be desirably utilized with the present invention.
The mechanical stripper of the invention may be used as the sole and continuously maintained primary stripping system for a copier, or as a downstream "back-up" stripper system to catch mis-strips from another type of stripping system such as a vacuum stripper, upstream therefrom. Such mis-strips may occur with sheets whose particular weight, humidity, curl, or other condition renders them particularly difficult to strip from the imaging surface by the primary stripping system.